A gas turbine engine such as a turbojet or turbofan engine powering an aircraft from takeoff through flight, approach, and landing produces noise from the air being compressed therein and from the air and combustion gases being discharged therefrom. Fans and compressors include at least one row of a plurality of circumferentially spaced apart rotor blades for compressing air channeled therethrough followed in turn by a row of circumferentially spaced apart stator vanes. The rotor blades rotate about a longitudinal centerline axis of the engine at a rotational speed N and effect a blade passing frequency (BPF) which is the product of the rotational speed N and the number B of rotor blades. Air channeled between the blades and vanes and inside the duct surrounding the blades and vanes generates conventionally known discrete frequency spinning mode tones or noises within the duct.
Spinning mode noise is conventionally known to include rotating pressure fields caused by both rotation of the rotor blades themselves, and by interaction of the rotor blades with adjacent stator vanes. The spinning mode tones are discharged from the engine either upstream through the duct inlet or downstream through the duct outlet, or both and are radiated toward the ground upon takeoff or landing of an aircraft being powered by the engine. The spinning mode tones occur at discrete frequencies including the fundamental blade passing frequency BPF, alternatively referred to herein as the first harmonic, and higher order frequencies including the second, third and higher harmonics.
In order to reduce the spinning mode noises, at takeoff or approach for example, it is known to selectively determine the number of vanes relative to the number of blades, preferentially space the vanes from the blades, and provide noise suppression liners along the inner wall of the duct surrounding the blades and vanes. These solutions decrease the magnitude of the spinning mode noises either at their inception, or after they are generated by being suitably absorbed into the suppression liner. In order to reduce the noise at its source, a conventionally known cut-off parameter, or ratio, is used wherein the values thereof less than 1.0 will effect decay of the noise, and values 1 or greater will effect propagation of the noise through the length of the duct with essentially undiminished intensity which will therefore radiate from the duct into the ambient air and toward the ground resulting in community noise.
In order to ensure the decay of spinning mode noise, it is conventionally known to select the number of vanes V to be greater than or equal to twice the product of the number of blades B and the blade passing frequency harmonic number (n). For example, to ensure decay of the spinning mode tone associated with the fundamental blade passing frequency BPF, i.e. harmonic number n=1, the number of vanes V should be greater than or equal to twice the number of blades B. And, to ensure decay of the spinning mode tones for the fundamental and second harmonic, the number of vanes V should be greater than or equal to four times the number of blades B.
However, for modern high by-pass turbofan engines requiring a relatively large number of rotor blades, the resulting number of stator vanes becomes impractically high. Accordingly, conventional practice is to select the number of stator vanes to cut-off the fundamental spinning mode noise at the fundamental blade passing frequency BPF only. Although the BPF tone levels are reduced, the higher harmonic tone levels are relatively high and the conventional sound suppression liners are less effective in attenuating these tones since the energy thereof is directed primarily along the engine longitudinal axis.